1. Field of the Invention
The present invention relates to a method for controlling a fusing operation and, more particularly, to a method including a control algorithm for minimizing temperature droop of a fuser at the beginning of a fusing operation.
2. Related Prior Art
In an electrophotographic image forming apparatus, such as a printer or copier, a latent image is formed on a light sensitive drum and developed with toner. The toner image is then transferred onto media, such as sheets of paper, and is subsequently passed through a fuser where heat and pressure are applied to melt and adhere the unfused toner to the surface of the media. There are a variety of devices to apply heat and pressure to the media such as radiant fusing, convection fusing, and contact fusing. Contact fusing typically comprises cooperating nip forming members including a heating member and a backup member, where the heating member may comprise a halogen lamp, an inductive heater or a ceramic heater. Fusers including inductive heaters and ceramic heaters generally include a belt backup member comprising a thin belt and a backup roll, and have a very low thermal mass between the heater and the paper conveyed through the fuser. Such fusing systems are generally capable of being heated relatively quickly from room temperature to a fusing temperature, and are capable of responding quickly to sudden changes in temperature during operation of the fuser.
Halogen lamp fusers generally include a hot roller cooperating with a backup roller to form a nip through which the toned media passes. The hot roller may comprise a metal core with a conductive rubber coating surrounded by a PFA or PTFE sleeve and a halogen lamp located inside the metal core. In order to minimize the cost of the fuser, it is desirable to provide the backup roller as an unheated roller. A temperature sensor may be located in contact with an outer surface of the hot roller to provide a temperature signal for controlling the temperature of the fusing operation to a predetermined target or set point temperature.
At the beginning of a print job, the hot roller may rotate against the backup roller causing an initial transfer of heat energy from the hot roller to the backup roller until the backup roller reaches an elevated temperature. The rotation of the rollers at the beginning of a print job may be a continuation of rotation of the rollers following a previous print job, or may occur following a stationary condition of the rollers. Due to a relatively large thermal mass associated with the hot roller, i.e., a high thermal mass in comparison to that of inductive and ceramic heater fusers, energy provided from the halogen lamp in response to a drop in temperature, as detected by the temperature sensor, will not reach the outer surface of the hot roller before a substantial temperature decrease or droop of the hot roller has occurred. For example, heat energy traveling from the interior of the hot roller may take 5 to 10 seconds to reach the outer surface of the hot roller. During initial printing of the first 2 to 3 pages of a print job, during which heat is removed from the hot roller by both the backup roller and substrates, the surface temperature of the hot roller may droop 5° C. to 10° C., potentially resulting in poor fuse grade of the toned images at the beginning of the print job, where the droop in surface temperature will typically be greater when rotation of the rollers is started from a stationary condition.
Further, the thermal mass of the hot roller may cause a delay in sensing the heat energy present in the hot roller, where sufficient heat energy to warm the surface of the hot roller to the target temperature may be present in the hot roller but undetected by the temperature sensor, resulting in extended application of power to the halogen lamp leading to a temperature overshoot, heating the hot roller substantially above the target temperature. A temperature overshoot condition may adversely affect the quality of the toned images, such as by causing toner offset, and additionally may result in a structural failure of the hot roller such as delamination of the rubber coating from the hot roller core.
Accordingly, there continues to be a need for a method for accurately controlling a two-roller single lamp fuser to a target temperature in a minimum amount of time, while minimizing the effects of thermal mass of the fuser rollers, including temperature droop and temperature overshoot at the beginning of a print job.